One known type of turbine comprises a rotor extending along a longitudinal axis; and a number of stages, each comprising a fixed stage and a movable stage. The fixed stage comprises a fixed inner ring; a fixed outer ring; and a number of so-called stator blades arranged radially between the inner ring and the outer ring, and fixed at one end to the inner ring, and at the other end to the outer ring. The movable stage comprises a number of so-called rotor blades arranged radially about the rotor and fixed to it by only one so-called base end.
Market demand in recent years has been for increasingly large steam turbines, to obtain high-efficiency, low-cost machines. More specifically, the tendency is towards increasing the size of the turbine exhaust section, i.e. the section at the final stage of the turbine, with the result that the final stage of a turbine of the type described above comprises extremely long stator blades, and extremely long rotor blades characterized by a marked twist along the blade axis. The twist provides for withstanding high pressure loads and large variations in steam flow speed, especially at the opposite end of each blade to the base.
The marked twist in the rotor blade, however, is not easy to produce, and involves considerable effort on the part of design engineers to minimize load losses along the rotor blade.
Moreover, large variations in steam flow speed, especially the tangential component, at the opposite end of the rotor blade to the base expose the surface of the rotor blade to serious damage by slow erosion caused by condensation dripping on the leading edge of the rotor blade.